CVE-2021-47389 in Linux
Summary
by MITRE • 05/21/2024
In the Linux kernel, the following vulnerability has been resolved:
KVM: SVM: fix missing sev_decommission in sev_receive_start
DECOMMISSION the current SEV context if binding an ASID fails after RECEIVE_START. Per AMD's SEV API, RECEIVE_START generates a new guest context and thus needs to be paired with DECOMMISSION:
The RECEIVE_START command is the only command other than the LAUNCH_START command that generates a new guest context and guest handle.
The missing DECOMMISSION can result in subsequent SEV launch failures, as the firmware leaks memory and might not able to allocate more SEV guest contexts in the future.
Note, LAUNCH_START suffered the same bug, but was previously fixed by commit 934002cd660b ("KVM: SVM: Call SEV Guest Decommission if ASID binding fails").
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Analysis
by VulDB Data Team • 07/10/2024
The vulnerability CVE-2021-47389 affects the Linux kernel's KVM implementation specifically within the SVM (Secure Virtual Machine) subsystem that handles AMD's SEV (Secure Encrypted Virtualization) technology. This security flaw resides in the hypervisor's handling of SEV guest context management during the RECEIVE_START operation, which is a critical component of the SEV API specification. The issue manifests when the system attempts to bind an ASID (Address Space Identifier) to a newly created SEV guest context but fails to properly decommission the previous context when this binding operation succeeds, creating a memory leak scenario in the hypervisor's SEV firmware interface.
The technical flaw stems from a missing DECOMMISSION command execution in the SEV_RECEIVE_START flow within the KVM SVM implementation. When the hypervisor processes a RECEIVE_START command, it creates a new guest context and handle as mandated by AMD's SEV specification, but fails to properly clean up the previous context when ASID binding operations fail. This oversight directly violates the SEV API contract which requires that every RECEIVE_START operation be paired with a corresponding DECOMMISSION command to maintain proper resource management. The vulnerability is particularly concerning because it directly impacts the hypervisor's ability to manage SEV guest contexts over time, as the firmware leaks memory resources that should be reclaimed through proper decommissioning procedures.
The operational impact of this vulnerability extends beyond simple resource leakage to potentially catastrophic system failures in virtualized environments utilizing SEV technology. When the firmware accumulates unreleased SEV guest contexts due to missing decommissioning, it eventually exhausts the available context slots in the SEV firmware, leading to subsequent SEV launch failures that can completely disable secure virtualization capabilities for affected systems. This creates a denial-of-service condition where legitimate virtual machines cannot be launched with SEV encryption enabled, effectively compromising both the availability and security posture of the virtualized infrastructure. The vulnerability affects systems running Linux kernels with KVM support and AMD processors with SEV capabilities, particularly those managing multiple virtual machines that require frequent SEV context switching.
This vulnerability aligns with CWE-404, which covers improper resource management, and specifically demonstrates poor handling of memory and context resources in virtualization environments. From an ATT&CK perspective, this represents a privilege escalation vector through resource exhaustion and a denial-of-service attack that can compromise system availability. The fix implemented in commit 934002cd660b addressed a similar issue in LAUNCH_START but failed to apply the same logic to RECEIVE_START, creating an asymmetric vulnerability in the SEV handling code. Organizations should apply the kernel patches that implement proper SEV context decommissioning for both RECEIVE_START and LAUNCH_START operations, ensuring that all SEV guest contexts are properly cleaned up regardless of ASID binding success or failure conditions. The mitigation requires updating to affected Linux kernel versions that contain the specific fix for the SVM SEV context management, which should be prioritized in security-conscious environments utilizing AMD SEV technology.